1. Field of the Invention
This invention relates to a continuous mixing machine having a superior mixing performance. More particularly, this invention relates to a continuous mixing machine having a plurality of rotors each provided with blade members and a mixing chamber encircling the plurality of rotors.
2. Description of the Prior Art
The known machines for mixing high molecular weight materials such as synthetic resins include, by category, a continuous mixing machine which includes a monoaxial or biaxial rotor and a mixing chamber encircling the rotor; a batch mixer, an example of which is the Banbury mixer, and a screw extruder. Of these machines, the biaxial continuous mixing machine provides the most efficient mixing because of its high shear speed which helps plasticize the material in a short period of time.
Thus, as shown qualitatively in the diagram of FIG. 1, the shear speed of a mixer is inversely proportional to the mixing time required for obtaining a mixed or kneaded product satisfying a given set of quality criteria, and the blocks (A), (B) and (C) of FIG. 1 represent the ranges of characteristics of the continuous mixer, batch mixer and screw mixer, respectively. In the case of a biaxial continuous mixer, the highest shear speed is 1000 sec..sup.-1, and at that speed the mixing time is about 10 to 40 seconds. Due to such a high shear speed, this type of machine accomplishes a thorough mixing of material in a short time to provide a remarkably high efficiency of operation and, at the same, enables one to build a compact machine for a given job. Furthermore, a continuous mixing machine of the described type displays a superior mixing performance when applied to the mixing of composite materials such as mixtures of synthetic resins with large amounts of inorganic fillers.
Particularly, with the recent development of the synthetic resin industry, resins have come to be used in an increasing assortment of uses. Thus, there have been developed not only new synthetic resins but various blend polymers made up of two or more different synthetic resins and composite materials composed of synthetic resin and various additives. Accordingly, many continuous mixing machines are now required to be variable in residence time, i.e. the time during which the material being mixed is retained within the mixer, so that the longer mixing times required for some types of materials may be provided. As far as such a continuous mixing machine is concerned, the degree of opening of the discharge orifice at the exit end of the machine and the hourly output capacity of the machine are important design considerations but certain problems remain yet to be solved in order to provide the variability of residence time over a sufficiently broad range to adjust for the divergent characteristics of a large variety of materials.
Thus, the operation parameters of this type of continuous mixing machine generally include hourly output, rotational speed of mixing rotors, the degree of opening of the discharge orifice and cooling conditions applicable to the mixing chamber and rotors, and with the output, rotor speed and cooling conditions being held constant, the temperature and the degree of mixing can be adjusted by controlling the degree of opening of the discharge orifice and the relationship between the degree of opening (.theta.) of the discharge orifice and the residence time of material within the machine as shown in FIG. 2. Thus, as the opening of the discharge orifice is decreased, the consequently increased discharge resistance results in an increased packing density of the material in the machine and a prolonged residence time of the material. The increased residence time means a higher degree of mixing and, hence, an increased temperature of the product. Further, as shown in the diagram of FIG. 2, taking as an example the cases of the outputs Q.sub.1 and Q.sub.2 (Q.sub.1 &gt;Q.sub.2), the residence time can also be increased by decreasing the output but such an approach causes a reduction in the efficiency of the mixer. The residence time may also be increased by increasing the length of the rotors. However, such an attempt will cause an increase in temperature of the material and where there is a limitation on the temperature of material, the degree of opening of the discharge orifice will have to be increased and, therefore, the residence time may not necessarily be increased.
Moreover, there are many cases in which it is more important to achieve uniform mixing than to increase the mean degree of mixing. The problem of fish-eyes in film grades of polyolefin resin and the problem associated with the addition of carbon black are cases in point. Thus, it is of course important to decrease the total number of fish-eyes or the total number of undispersed carbon black particles but if the fish-eyes or carbon particles in excess of certain dimensions are left over even if they are small in quantity, the properties of mixed products will be adversely affected. To prevent such results, it is necessary to ensure a uniform residence time of material within the mixing machine and to preclude the short-pass problem, i.e. the passage of a localized portion of material through the machine substantially without being subjected to shearing work. The conventional continuous mixing machine of this type has this problem of short-pass and, hence, the disadvantage of a variation of residence time.